The present invention relates to a method for replacing airfoils on turbine vanes, and more particularly, to a method of replacing airfoils such that they can be upgraded and/or repositioned within a turbine vane platform.
Components of gas turbine engines, especially those positioned within the hot section of the engine, are exposed to a harsh operating environment. Extreme operating temperatures, accompanied by repeated temperature cycling during engine warm-up, operation and cool-down can quickly deteriorate engine components. These components include HPT (high-pressure turbine) vane segments which can become damaged or worn such that they require repair or replacement.
A turbine engine vane segment is typically comprised of an outer and inner platform, between which one or more airfoils are positioned. The airfoils are either cast as a single unit with one or both of the platforms or are separately welded or brazed to the platforms in the form of a component assembly. Some turbine vanes are complex castings, comprising two, three or more airfoils integrally cast to the inner and outer platforms. Another form of turbine vanes is paired assemblies. A paired assembly is a vane in which a single airfoil is integrally cast between two platforms. Two of these castings are brazed or welded along mateface joints to create a doublet vane assembly.
It would be desirable to be able to modify these vanes from a single casting or welded or brazed pair to a multi-piece assembly consisting of individual airfoil segments attached to inner and outer platforms to facilitate subsequent vane repairs and airfoil replacement.
Advances in material science often provide improved materials for use as airfoil members, and may provide airfoils having sizes which differ from those used in existing vanes. In addition, airfoil positioning within the vane, i.e. location on the vane segment platform, might require adjustment. For example, an adjustment to the nozzle opening area between adjacent vanes (hereinafter the "class area") may be required or desirable.
Components in gas turbine engines are air cooled and are fabricated from expensive materials. These components are also costly to assemble. As a result, it is desired to be able to efficiently repair the damage, while providing for upgraded components and materials within each vane, such that as much of the original materials as possible can be reused.
Conventional airfoil replacement repair procedures involve separating the platforms by cutting the airfoils therefrom. This procedure retains a stub on each platform where the airfoils are cut out. The replacement airfoils are then typically welded to the stubs using electron beam welding techniques. Because the new airfoils must be positioned on the existing stubs, the positioning of the new airfoils is restricted. Also, because airfoil stubs are retained, complete refurbishment of the platform gaspath surfaces by an automated process is not possible. The irregularly contoured stub protruding from each platform requires that brazing and contouring of the platforms be done by hand. It is desirable, however, to automate as many refurbishment operations as possible in order to minimize repair prices and time.
Another airfoil replacement procedure has been applied to HPT turbine vanes that were originally created as component assemblies as described above, in which the airfoil castings are brazed into pockets in the separately cast platforms. When repairing a component assembly vane, the entire assembly can be heated to a temperature sufficient to melt the brazed joint, allowing the components to be pulled apart. This procedure has been described in U.S. Pat. No. 5,444,911 to Goodwater et al. Alternatively, the airfoil can be cut from the platforms, followed by subsequent machining operations to restore the platform pocket. This alternative procedure has been described in U.S. Pat. No. 5,813,832 issued to Rasch et al. However, using either procedure, the replacement airfoil is limited to placement within the existing socket in the platform, thereby preventing the relocation of airfoils, i.e. class size alteration, or the use of airfoils which are of a different size from the original.